1. Field of the Invention
The invention relates to a vehicle position identifying system and a vehicle position identifying method.
2. Description of Related Art
As a method of identifying the position of a vehicle, there is available radio navigation using a global positioning system (GPS) and self-contained navigation using the speed and direction of the vehicle. In the radio navigation using the GPS, an absolute position coordinate including latitude and longitude is obtained as the position of the vehicle. In the self-contained navigation, by calculating a moving distance and a moving direction from a reference position using the vehicle speed and direction, a relative position coordinate from the reference position is obtained as the position of the vehicle. At present, both of these kinds of navigation are used for identification of the position of the vehicle.
Further, for the identification of the position of the vehicle, in addition to these kinds of navigation, for example, so-called map matching for matching a traveling track of the vehicle with the shape of a road indicated by road map information is also performed. In the map matching, when the traveling track does not match the road shape, accuracy of the position of the vehicle is improved by correcting the vehicle position to an appropriate position on the road.
On the other hand, in the radio navigation using the GPS, positioning accuracy is sometimes deteriorated because, for example, an error of about several meters to less than twenty meters occurs in the obtained absolute position coordinate or a reception state of a radio wave is deteriorated among high-rise buildings, in a mountain area, or the like. In the self-contained navigation, although an error per unit moving distance is smaller than a positioning error by the GPS, because of characteristics of the self-contained navigation, the error tends to accumulate. Therefore, the error increases as the traveling distance increases. In the map matching, although identification accuracy for the vehicle position can be improved at the time of a right turn, a left turn, or the like at an intersection, concerning position accuracy in the front back direction of the road, steady increase in identification accuracy of the vehicle position cannot be expected.
Therefore, in recent years, to make up for the disadvantages of these methods, there has also been proposed, for example, an apparatus that determines whether a moving distance of a vehicle from a reference position is within a positioning error range included in positioning data received from a GPS (see, for example, Japanese Patent Application Publication No. 8-334335 (JP 8-334335 A)). The apparatus calculates a relative position to the last host vehicle position using the self-contained navigation and obtains positioning data including absolute position information and positioning degree difference range information using the radio navigation. When the moving distance of the vehicle from the last vehicle position is within the positioning error range, the apparatus sets the calculated relative position as a host vehicle position. When the moving distance from the last host vehicle position is outside the positioning error range, the apparatus sets, as the host vehicle position, a position based on absolute position information obtained by the radio navigation.
Incidentally, as explained above, not only factors on the GPS side but also factors of a road environment in which the vehicle travels greatly contribute to the positioning error by the GPS. However, the positioning error range included in the positioning data does not take into account factors concerning a traveling environment of the vehicle such as the road environment. Therefore, even in the apparatus, there is still room for improvement concerning identification accuracy itself for the position of the vehicle.